In order to protect such circuit arrangements against overloading use is frequently made of non-linear negative feedback between the input and the output of the amplifier. As an example, U.S. Pat. No. 4,006,428 describes a circuit arrangement which, in order to avoid distortion, employs non-linear negative feedback, which enables the operating point of the amplifier to be shifted so that the amplifier always operates within the linear operating range. Swedish patent application No. 7301979-6, which has been laid open to public inspection, also reveals a similar step, the object being to prevent excess voltages from appearing on the amplifier, which would otherwise damage the amplifier.
The invention is concerned with a short-circuit protection problem. In this respect it is not so much of interest that excessive signal or operating voltages appear on the amplifier itself, but that an excessive current would flow through the load impedance to be connected to the amplifier, so that said load could be damaged.
Especially in output amplifiers for car radios this risk of short-circuiting is an essential problem. The relevant output amplifier generally takes the form of an integrated circuit with two amplifier sections operating in a bridge arrangement, between whose outputs a load, specifically a loudspeaker, is included. If owing to carelessness or wiring defects one of these outputs comes into contact with the metal parts of the car (earth), a direct current of several amperes could permanently flow through this loudspeaker, which loudspeaker is generally not capable of handling such current.